As is well known, conventional solid polymer electrolyte fuel cells are formed by stacking unit cells. The unit cell is formed by a solid polymer electrolyte membrane formed by an ion exchange membrane, electrodes disposed on both sides of the solid polymer electrolyte membrane, and a separator with grooves (groove pattern) for supplying a fuel gas and an oxidizer gas to each of the electrodes.
The unit cell generates an electric power by electrochemical reaction between the fuel gas supplied to an anode electrode side and the oxidizer gas supplied to a cathode electrode side, and then the unit cell provides the electric power to an exterior device. Thus the separator needs to have a performance for separating the fuel gas from the oxidizer gas and sufficient strength not to be damaged in order not to leak both gases.
To improve the strength of the separator, a method for producing the separator has been improved in the development for fuel cells. For example, the separator is made of a metal plate or a carbon plate formed by a cutting machine.
Since it is very expensive to produce the separators by machining the metal plate or the carbon plate, the separator has been produced by a compression molding or an injection molding with a resin and carbon particles instead of machining. The separator produced by compression molding or injection molding is lower in cost and is higher in strength compared to the separator produced by machining.
For example, according to a Japanese Patent Application Publication published on Sep. 8, 2000, as Toku-Kai 2000-243409, the compression molding is employed as the method for producing the separator. The cost of compression molding is lower than the machining, but the process of compression molding takes a longer time due to warm and cool die assembly and curing time for the resin (at least 5 minutes), the productivity of the separator is therefore not efficient. The method by the compression molding has problems in the dimensional accuracy and flatness of the separator resulting from burrs (or flash) by molding and various thicknesses.
According to another Japanese Patent Application Publication published on Nov. 30, 2000, as Toku-Kai 2000-331690, the injection molding is employed as the method for producing the separator. The injection molding dose not take a longer process time for cooling the die assembly and curing time for the resin, the productivity of the injection molding is more improved than that of the compression molding method. But, in the injection molding, material to be molded needs to have high fluidity in the die assembly. Thus the material for the injection molding needs to contain much resin as a binder for binding carbon particles. As the separator, containing the much resin is not preferable because the conductivity of the entire separator is decreased. The prerequisite area of the separator is decided by the electric current in the stacks of the fuel cell. According to the fuel cell for the vehicle, the stacks need to be minimized in the thickness direction. To keep the fluidity of the material, minimizing the stacks in the thickness direction is difficult.